What happens if data write overlaps with underflow when using R1H or R2H ?

Question:

(Timer) There is a note stating, "When writing data to each reload register R1H or R2H of timer 1 or timer 2 while timer 1 or timer 2 is operating, avoid a timing when timer 1 or timer 2 underflows." What happens if the data write overlaps with underflow? [2006/01/16]